Conventionally, a semiconductor light emitting device using a gallium nitride based compound semiconductor for emitting blue color type light (yellow from ultraviolet rays), for example, has had a structure shown in FIG. 7. More specifically, the semiconductor light emitting device comprises a low-temperature buffer layer 22 made of GaN on a sapphire substrate 21, for example, an n-type layer (a clad layer) 23 in which n-type GaN is epitaxially grown at a high temperature, an active layer 24 formed of a compound semiconductor made of a material for defining a light emitting wavelength to have a smaller band gap energy than the band gap energy of the clad layer, for example, an InGaN based compound semiconductor (which means that a ratio of In to Ga can be variously changed and so is the following), and a p-type layer (a clad layer) 25 including a p-type AlGaN based compound semiconductor layer 25a (which means that a ratio of Al to Ga can be variously changed and so is the following) and a GaN layer 25b, and a p-side electrode 28 is provided on the surface of the GaN layer 25b and an n-side electrode 29 is provided on the surface of the n-type layer 23 which is exposed by etching a part of the provided semiconductor layers. In some cases, the AlGaN based compound semiconductor layer is used on the active layer 23 side of the n-type layer 23 in the same manner as the p-type layer 25 in order to enhance the confinement effects of a carrier.
As described above, the conventional blue color type semiconductor light emitting device using the gallium nitride based compound semiconductor is formed by providing gallium nitride based compound semiconductors such as GaN, InGaN based and AlGaN based compound semiconductors which form a light emitting layer on the sapphire substrate. However, the lattice constant of the sapphire substrate is different from that of the gallium nitride based compound semiconductor by about 16%. Therefore, it is impossible to obtain a gallium nitride based compound semiconductor layer having excellent crystalline properties. In order to solve this problem, a buffer layer such as GaN, AlN or the like which is formed at a low temperature is provided between a single crystal layer of the gallium nitride based compound semiconductor forming a light emitting layer and a substrate, thereby enhancing the crystalline properties of the gallium nitride based compound semiconductor layer as described above.
Although the crystalline properties of the light emitting layer can be improved and the blue color type semiconductor light emitting device can be utilized by providing the above-mentioned buffer layer to be formed at a low temperature, the problem of the crystal defect of the gallium nitride based compound semiconductor layer has not completely been solved. There has been a problem in that a luminance cannot fully be enhanced due to a great leakage current and yield cannot be enhanced due to an insufficient luminance obtained by a slight change in a manufacturing process.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a semiconductor light emitting device having gallium nitride based compound semiconductor layers provided in which crystal defects based on a difference in a lattice constant can be decreased, a leakage current can be reduced and an excellent light emitting efficiency can be obtained.
It is another object of the present invention to provide a method for manufacturing a semiconductor light emitting device in which the crystal defects can be decreased based on the difference in the lattice constant when the gallium nitride based compound semiconductor layers are to be provided.